Thieves and other unauthorized entrants most often attempt to disable a door lock by various simple techniques that require little time. Rather than attempting to retract the lock bolt by actuating the lock mechanism, they sometimes resort to a direct attack on the bolt itself. This technique is most frequently employed where the construction of the door frame and walls surrounding the lock is such that the extended bolt is relatively accessible.
One such technique calls for punching a hole in the wall adjacent the swingable edge of the door so that a thin metal rod inserted through the hole rests against the outer end of the bolt. A sharp hammer blow is then applied to the rod, driving the bolt back into the lock mechanism. Many otherwise secure locks can be defeated with relative ease in this manner because the entire force of the blow must be absorbed by the actuator mechanism and the linkage by which that mechanism is connected to the bolt. Space limitations and the complexity of the mechanism often dictate that it must have considerably less strength than the bolt itself and it is often found that the pin by which the inner end of a bolt link is attached to the actuator mechanism can be sheared off by a relatively small force, allowing the bolt to be completely retracted into the door.
Another technique for attacking the bolt directly is to force a chisel between the door and the frame so that it contacts the bolt at a slight angle. A hammer blow is then applied to the chisel to drive the bolt back against the lock mechanism with the same result described above. This method of attack is most often used with an outwardly opening door where there is no overlap between the edge of the door and the frame and the bolt is, therefore, readily accessible. It should be noted that in many locks the bolt need be driven only a very short distance into the lock before one of the lock components fails and the bolt can slide freely into its retracted position.
In the case of an inwardly opening door, perhaps the simplest technique for defeating a lock is a powerful shoulder blow or kick to the door itself. It is sometimes found that when this technique is used, the bolt binds within a socket in the door frame and, as the door tends to open, the bolt is pulled away from the lock. Although the bolt may be capable of withstanding large purely transverse forces, the actuator mechanism sometimes fails under these circumstances in such a way that the bolt is completely removed from the lock and remains with the frame as the door swings open.
In an effort to combat lock failures of the type described above, many lock manufacturers have been forced to employ relatively heavy and expensive actuator mechanisms. They have generally avoided the use of plastics and other lighter, less expensive materials that would be suitable for producing the desired bolt movement when the lock is operated, but could not withstand forces that would tend to push the bolt into the lock or pull the bolt out of the lock.
A principal objective of the present invention is to provide a door lock which utilizes a relatively simple actuator mechanism and can incorporate plastic or other lightweight components but is nevertheless relatively immune to attack by forces applied to the bolt.